This invention relates to a card assembly having a loop antenna formed of a bare conductor and a method for manufacturing the card assembly. More particularly, this invention relates to a card assembly, such as a smart card or the like, having a bare conductor loop antenna with at least one overlapping section of the loop.
Insulated copper wire is typically used to form multi-turn loop antennas for smart cards or the like. The electrical insulation of the insulated copper wire is typically applied using drawing or spraying methods, which creates a thin dielectric sheathing with some inherent porosity. A multi-turn loop antenna overlaps itself at least once at a cross-over region. The cross-over region is usually exposed to heat and pressure when multiple polymeric layers are laminated together to form a smart card. The exposure to heat and pressure may damage the relatively thin electrical insulation of the insulated wire. Consequently, a loop antenna with damaged insulation is prone to electrical shorting at the cross-over region. Electrical shorting results in an impedance mismatch between the loop antenna and the internal circuitry of the smart card. The impedance mismatch often leads to catastrophic failure of the smart card.
Several methods have been used to strip the insulation of the insulated wire at the ends of the loop antenna to make the ends receptive to an electrical connection to the internal circuitry of the smart card. Stripping methods may be generally categorized as mechanical, chemical, or thermal.
Mechanical stripping includes milling procedures, mechanical abrasion, and sand blasting. Mechanical stripping is often difficult to precisely control, particularly where wires have diameters of less than 8 mils. In addition, mechanical stripping produces debris which must be removed by vacuuming or other imperfect cleaning procedures.
Insulated copper wire may be milled to remove the insulation at its ends, after the insulated copper wire is embedded into a polymeric layer of the smart-card assembly. Ideally, wires with generally circular cross-sections are milled to produce wires with generally semi-circular cross sections. The rectilinear or flat portion of the milled wires readily accept a conductive medium (i.e. conductive adhesive) to form an electrical connection. However, milling the wires in such a manner may damage or destroy the fragile wires or leave contaminating residue from the milling process. The contamination residue may include polymeric smears from the insulation and metallic particles from the ends. Accordingly, milling of the wires may result in smart cards with intermittent or open electrical connections to the loop antenna.
Solvents are representative of typical chemical treatments from stripping the insulation. Ideally, solvents would selectively chemically attack the insulation without detrimentally affecting the surrounding polymeric layer of the card assembly. However, in practice the solvent may affect the structural integrity of the polymeric layer or cause a gummy residue from the insulation to remain on the ends of the loop antenna. Thus, dissolving the insulation with chemical solvents may result in an intermittent or open connection to the loop antenna caused by contamination from the gummy residue.
Thermal stripping uses heat to selectively remove the insulation from insulated wire. Thermal stripping may use a laser, which is targeted at the insulated wire, but yet secondarily heats the smart card. The laser often heats the smart card sufficiently to deform or warp the polymeric base of the smart card.
Thus, a need exists for a card assembly having a loop antenna with a reliable impedance, which is generally immune to electrical shorting at a cross-over region. Furthermore, a need exists for a loop antenna which does not require stripping of insulation, or is otherwise receptive to an electrically and mechanically sound connection to the internal circuitry of the card.